1. Field of the Invention
The present invention relates to an MRI apparatus and a high-frequency coil for the same, which enable imaging an object to be imaged with a non-cylindrical shape.
2. Description of the Related Art
Magnetic resonance imaging (MRI) is a method in which atomic nuclei spin of a living tissue set in a static magnetic field are excited by a high-frequency signal having a Larmor frequency, and image data is reconstructed on the basis of a magnetic resonance signal generated by the excitation. MRI apparatuses are medical diagnostic apparatuses which obtain diagnostic information by the MRI method. MRI apparatuses are useful in the field of medical diagnosis of today, since they can obtain many kinds of information, for example, not only anatomical information, but also biochemical information and functional information.
To generate image data of high image quality by MRI apparatuses, it is required to efficiently detect a faint MR signal generated in a living body, and many technical contrivances have been made therefore. As high-frequency coils used for detecting MR signals, different types of coils are used according to the direction of the static magnetic field. If the direction of the static magnetic field is horizontal, a saddle coil is used. If the direction of the static magnetic field is vertical, a solenoid coil is used. Although there are cases where a high frequency coil for transmitting a high-frequency signal and a high-frequency coil for receiving an MR signal are provided individually, it is possible to use the same high-frequency coil for the transmission and the reception, since timing of the transmission is different from timing of the reception.
On the other hand, volume coils of QD method are known as high-frequency coils with high sensitivity. Volume coils of birdcage type are generally used for horizontal static magnetic fields. A volume coil of birdcage type has two end rings and at least four rods (elements). The rods are arranged in a substantially parallel state in a direction of the static magnetic field (horizontal direction), and both ends of each rod are connected to the two respective end rings. In particular, birdcage volume coils having four rods are also called “slotted tube resonator”. A cylindrical imaging region having a relatively uniform high-frequency magnetic field distribution is formed inside such a volume coil.
In the meantime, if an imaging object is imaged in a high-frequency coil of birdcage type, it is desired that each rod is disposed close to a surface of the imaging object as much as possible. However, in the case of imaging an imaging object with a non-cylindrical shape, such as a region including a shank and a foot, it is required to use a nigh-frequency coil having an internal diameter corresponding to the size of the foot and therefore the shank is distant from the rods. If the high-frequency coil is used as an irradiation coil in such a situation, a larger transmission power is required in comparison with the case of using a high frequency coil having a smaller internal diameter. Further, if the high-frequency coil is used as a reception coil in this situation, the S/N ratio of a detected MR signal deteriorates.
On the other hand, a method of using a surface coil has been developed as a method of detecting MR signals with a good sensitivity. However, since sensitivity distribution in a surface coil is generally nonuniform and is not easily corrected, it is difficult to obtain image data of excellent quality from a surface coil.
As a method of solving the above problem, U.S. Pat. No. 5,277,183 discloses a cylindrical high-frequency coil 500 as shown in FIG. 15. The cylindrical high-frequency coil 500 includes end rings 504 each having an internal diameter corresponding to the thickness of the shank 501. Further, a part of the rods 503 is bent outside in accordance with the shape of a foot 502. In the cylindrical high-frequency coil 500, it is possible to bring the rods 503 close to the whole region of the shank 501 and the foot 502.
However, in the cylindrical high-frequency coil 500, a spatial distribution of a high-frequency magnetic field formed by current flowing through the rods 503 is nonuniform in a region formed by bending the rods 503. Nonuniformity in the high-frequency magnetic field affects the quality of the image data reconstructed.